NTSB report cites faulty weld on San Bruno pipe

Published 4:00 am, Saturday, January 22, 2011

Photo: Michael Macor, The Chronicle

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A firefighter at the blast site where the hugh natural gas line in now exposed inside a large crater on Friday Sept. 10, 2010, in which an explosion and fire leveled the surrounding neighborhood the night before in San Bruno, Calif.
Ran on: 09-11-2010
A firefighter at the San Bruno blast site views the natural-gas pipeline now exposed inside the crater the explosion caused.
Ran on: 09-24-2010
A firefighter works at the site of the San Bruno gas line blast. Investigators are looking at microbes as a possible cause.
Ran on: 01-14-2011
The blast site in San Bruno where the huge natural gas pipeline exploded in September.
Ran on: 06-29-2011
A severed gas line sits at the site of the blast. An expert says records suggest fluctuating pressure could have weakened it.
Ran on: 06-29-2011
A severed gas line sits at the site of the blast. An expert says records suggest fluctuating pressure could have weakened it. less

A firefighter at the blast site where the hugh natural gas line in now exposed inside a large crater on Friday Sept. 10, 2010, in which an explosion and fire leveled the surrounding neighborhood the night ... more

Photo: Michael Macor, The Chronicle

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NTSB report cites faulty weld on San Bruno pipe

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An inferior weld on the seam of the San Bruno gas pipeline was overstressed to the breaking point at the time of the line's deadly explosion, according to a detailed scientific analysis released Friday by federal transportation safety officials.

The metallurgical report shed new light on the Sept. 9 blast that killed eight people and destroyed 38 homes, identifying for the first time a particular seam weld as the site of the rupture. It found that the weld was half as thick as it should have been.

Experts say such an obvious flaw would have been exposed during the kind of rigorous high-pressure water testing inspections that the utility had long ruled out as expensive and burdensome.

The incomplete weld, experts say, was so inferior as to be substandard even at the time of the pipe's installation in 1956. Its vulnerability casts further doubt on the validity of the already-questioned pressure peak level, set by PG&E at 400 pounds per square inch, on the line.

PG&E's records - shown by investigators to be in error - indicated that the line was seamless, when in fact the San Bruno site was part of a cobbled assortment of potentially inferior 4-foot-long seamed pipes of apparently unknown origin. Records for hundreds of miles of pipelines elsewhere in PG&E's system are now under scrutiny and regulators might compel the use of high-pressure water testing to seek out similar weld flaws.

The problem seam weld in San Bruno, Friday's National Transportation Safety Board report noted, developed a crack "consistent with ductile overstress from the root of the weld." Ductile refers to the action of pulling,experts say, and the force eventually ripped apart the line at the seam at a time when pressure surged to 386 pounds per square inch. The pressure surged because of a malfunction of the pipeline system's power source before the San Bruno explosion.

The report noted that there were several circumferential, or girth, welds, and all of them had similar flaws. One of them also failed in the accident, but the report did not specify the cause of the "overstress" on those welds. Seam welds run along the length of the pipes.

'Horrible welding'

UC Berkeley engineering Professor Robert Bea, reviewing the government's report for The Chronicle, concluded that the seam weld was so weak on the 3/8-inch-thick pipe that a disaster could have occurred at any time. He pointed to photos in the report as evidence that pressure surges over time - including two pressure spikes PG&E ran in 2003 and 2008 as reported in The Chronicle - could have accelerated cracking.

"Something is causing the crack to propagate - that may be a change in pressure," Bea said. "When the metal gets so thin, just a little bit of stress is enough to exceed the ability of the steel to withstand the pressure.

"It starts to stretch, like when you are blowing up a balloon - at some point you stretch it to the point it pops. That is what likely happened here."

He noted that the problem could be traced to when the line was installed in 1956. "It's not bad welding, it's horrible welding," said Bea, a trained welder.

Based on what he knew of the welds from reading the report, he said, the legal maximum pressure on the line should have been "zero."

He noted that it does not make any conclusions about why the weld failed, whether pressure surges had weakened it over time or whether "there were outside forces" that caused the stress on the line and the rupture.

Meanwhile, PG&E President Chris Johns issued a statement Friday calling the report "another important step" and praising the NTSB's "meticulous and painstaking work" in the investigation.

He pointed to the 20 percent reduction in pressure now in place on lines similar to the one in San Bruno as "a measure that builds a significant additional margin of safety into our current operating conditions."

Faulty records

The report's release followed recent revelations that PG&E had used faulty records to justify the San Bruno pipeline's legal maximum level of 400 pounds per square inch, and that it had been spiking the line to that now dubious level every five years. Ten other unidentified lines have been similarly spiked since 2003.

Bea and other experts say those spikes could have stressed any flaws over time, making them more vulnerable to failure when the pressure surged because of equipment malfunction to 386 pounds per square inch on Sept. 9. They pointed to the analysis released Friday as possibly making that case.

The utility, however, has defended the increases as preserving "operational flexibility" but announced it was putting them on hold pending the outcome of the federal investigation of the San Bruno rupture.

One conclusion that experts said is inescapable about the report: Only a high-pressure water, or hydrostatic, test would have been able to spot the welding flaws now being pointed to as the probable root of the blast. The utility had insisted such tests were impractical.

He said the report underscores the need to know what kind of problems exist in lines underground. "This thing lasted for 50 years," he said. "But if you have seam welds, you don't spike the pressure, you keep them under control."

Rep. Jackie Speier, D-Hillsborough, said Friday that the report was very disturbing and gave strong indications that the explosion was preventable.

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"The welds being inadequate by as much as 50 percent is a huge defect," she said. "The question is, at how many other locations along this system was this kind of sloppy and dangerous workmanship going on?

"What this report tells us, in the most tragic of terms, is that had PG&E done the proper kind of inspection (at the San Bruno site), they would have seen the flaws in the weld and they could have repaired them."

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